Studies in the Umbilicaria vellea group (Umbilicariaceae) in North America

The name Umbilicaria vellea has been applied to several taxa, but herein the taxon is circumscribed as a Holarctic species of Europe and North America with two different types of rhizinomorphs occurring in two layers. In more southern areas of North America a second taxon with one variable type of rhizinomorph and typically larger thalli is present. We propose the erection of U. americana to accommodate the previously unrecognized taxon. It further differs from U. vellea in that the lower cortex soon turns shiny black whereas in U. vellea the lower cortex remains light for a long time before turning dull black. A key is provided to assist in distinguishing these two species from other related species. Umbilicariae are particularly difficult to interpret because multiple individual thalli covering a range of ages must be collected to understand the full developmental sequence.